Process for joining a gas diffusion layer to a separator plate

ABSTRACT

There is provided a process for joining a gas diffusion layer to a separator plate of an electrochemical cell. The gas diffusion layer comprises a porous body that allows a reactant gas to diffuse through the gas diffusion layer. The separator plate comprises at least one landing surface formed on a surface of the separator plate, and the separator plate and landing surface comprising a polymer and conductive filler. The process includes the step of welding the landing surface to the gas diffusion layer by impregnating some of the polymer on the landing surface within a portion of the porous body.

FIELD OF THE INVENTION

The invention relates to a process for joining a gas diffusion layer toa flow field separator plate in an electrochemical cell to form anintegrated cell component, and in particular to a process for joininggas diffusion layers to flow field separator plates using resistance orvibrational welding.

BACKGROUND OF THE INVENTION

Electrochemical cells, and in particular fuel cells, have great futurepotential. Electrochemical cells comprising polymer electrolyte membrane(PEMs) may be operated as fuel cells wherein a fuel and an oxidant areelectrochemically converted at the cell electrodes to produce electricalpower, or as electrolyzers wherein an external electrical current ispassed between the cell electrodes, typically through water, resultingin the generation of hydrogen and oxygen at the respective electrodes ofthe cell.

FIG. 1 a illustrates a typical PEM electrochemical cell. Each cellcomprises a membrane electrode assembly (MEA) disposed between a pair ofseparator plates 5. The MEA comprises a catalyst-coated membrane 10interposed between a pair of fluid distribution layers 15, which aretypically porous and electrically conductive. The fluid distributionlayers 15 are commonly referred to as gas diffusion layers (GDLs). Thecatalyst-coated membrane 10 comprises an electrocatalyst on both sidesfor promoting the desired electrochemical reaction. The electrocatalystgenerally defines the electrochemically active area of the cell. The MEAis typically consolidated as a bonded laminated assembly of catalystcoated membrane and gas diffusion layers or an unbonded assembly, wherethe catalyst-coated membrane 10 is sandwiched between two GDLs 15 and iscompressed to form the MEA.

The cell separator plates 5 are typically manufactured from graphite orelectrically conductive plastic composite materials such as graphitecomposite made of graphite powders and graphite fibers held together bypolymer resin materials. Fluid flow spaces, such as passages orchambers, are provided between the separator plate 5 and the adjacentGDL 15 to facilitate access of reactants to the catalyst layer throughthe GDL 15, and facilitate removal of reaction by-products. Such spacesare formed more commonly as channels in the face of the separator plate5 that abuts the GDL 15. Separator plates 5 comprising such channels arecommonly referred to as flow field plates. In conventional PEM cells,the ribs of these channels, commonly termed as “landings”, function asthe electrical contact between the flow field plate 5 and GDL 15.Resilient gaskets or seals are typically provided between the faces ofthe MEA and each separator plate 5 around the periphery of the plates 5to prevent leakage of fluid reactant and product streams.

The GDLs 15 are typically made of porous, electrically conductivenon-corrosive material, such as carbon cloth or carbon paper. The GDLs15 provide uniform fuel and oxidant distribution to the catalyst-coatedmembrane 10 and facilitates the transport of product water from thecatalyst layers to the flow field plates 5. The GDLs 15 also provideelectrical contact between the catalyst layers and flow field plates 5,which helps in the harvesting of electrons from the electrochemicalreaction in the electrocatalyst layers.

The morphology, composition, porosity, tortuosity, thickness andcompression ratio of the GDL 15 impact the overall performance of theelectrochemical cell under different operating conditions. The natureand extent of contact between the GDLs 15 and the flow field plates 5significantly contributes to the overall performance of theelectrochemical cells. Good contact between the GDLs 15 and flow fieldplates 5 results in optimum cell performance by decreasing the resistiveloss between the GDLs 15 and the flow field plates 5.

PEM electrochemical cells are advantageously stacked to form anelectrochemical stack (see FIG. 1 b) comprising a plurality of cellsdisposed between a pair of end plates 20. A compression mechanism (notshown) is employed to hold the plurality of cells tightly together, tomaintain good electrical contact between the cell components, such asthe plates 5 and GDLs 15, and to compress the seals. The stackcompression force controls the nature and extent of contact between theGDLs 15 and the flow field plates 5. While a high stack compressionforce may provide good contact between the GDLs 15 and flow field plates5, it often can cause local damage to the physical structure of the GDLs15. A high stack compression force can also change the morphology of theporous GDLs 15 and impede the flow of oxidant and fuel to the catalystlayers. This impediment can lead to starvation at the reactive sites onthe catalyst layers and a resultant decrease in the performance of theelectrochemical cell.

The non-uniform distribution of reactants across the active area of thecell may also cause differential reaction zones, leading to hot-zonesbeing formed in the active area of the MEA. These hot-zones can thencreate pinholes in the membrane, resulting in the premature failure ofthe MEA.

Parts of the GDLs 15 may also sink into the flow field channels of theplates 5 when a high stack compression force is applied, resulting inadded stress to the GDLs 15 and landing junctions. This leads to arestriction of the flow of reactants and products through the channels,which affects the overall performance of the electrochemical cell.

In addition, the uneven surfaces of the flow field plates 5, and theuneven landing surfaces of the flow field channels may cause unevencontact between the GDL 15 and the flow field plate 5. The thicknessvariation of the GDL 15 can introduce pressure differentials across theactive electrochemical area of the MEA, resulting in a contactdifference with the landing surfaces of the flow field plate 5. Thisuneven contact results in lower conductivity between the GDL 15 and flowfield plate 5. It can also result in localized deformation of the GDL15. Therefore, in such a stack unit, if the electrical contactresistance at the interface between the flow field plate 5 and the GDL15 is large, the voltage drop is correspondingly large when a current ispassed in the stacking direction, leading to a lower electricalefficiency in the electrochemical cell stack.

Typically, an assembled cell stack is subjected to high compressionforce applied through the end plates to increase the contact between theGDL and separator plates in an electrochemical cell. A method forcompressing individual PEM cells within a stack for increasing theconductivity between the GDL and separator plates is disclosed in U.S.Pat. Nos. 5,534,362 and 5,736,269. These two patents describe a methodof compressing PEM cells together, wherein the compression pressure issimultaneously produced in each cell by a pressurized fluid.

Attempts have also been made to increase the contact area between theGDL and landing surfaces of the flow field plates to reduce theresistive loss between these two layers. For example, U.S. Pat. No.6,348,279 proposes a method to roughen the landing surfaces of the flowfield plates to increase the overall area of contact and facilitatepenetration of the roughened areas into the pores of the GDL to reducethe resistive loss and enhance the conductivity between flow field plateand GDL. However, the result of this process is largely dependent on thecompression force applied to the stack, which determines the degree ofpenetration of the roughened plate landing surface into the pores ofGDL. This therefore provides very little advantage over a non-roughenedlanding surface, as the overall stack conductivity remains dependent onthe stack compression force.

Another common method disclosed in the prior art involves thecorrugation of the actual separator plate itself as taught in U.S. Pat.No. 5,232,792. This method reduces electrical resistance by penetrationof the corrugated plate surface into the pores of the GDL thus improvingthe electrical conductivity between the GDL and flow field plate. Thismethod, however, is also dependent on the compression force applied tothe stack.

U.S. Pat. No. 5,049,458 teaches the application of concave and convexportions to the surface of the separator plate to create a “wave-form”or dimpled corrugation pattern with flat electrodes. The dimples arehemispherical in shape and tangentially contact the flat electrode atthe curved portion of the dimples and thus create good contact betweenthe plate and the electrode.

To decrease the resistive loss between the separator plate and GDL,separator plates possessing unique dimple configurations have beenproposed in U.S. Pat. No. 5,795,665. Each separator plate is formed withrows of dimples such that the dimples in successive rows protrude fromthe separator plates in opposing direction. The first separator plateabuts a first face of the PEM cell so that the dimples of the plateprotruding in a first direction abut the dimples of the PEM cellprotruding in the opposite or second direction. This opposite effect issaid to create good contact between the GDL and the separator plate.

A unit combining a separator plate and a GDL is disclosed in U.S. Pat.No. 6,280,870. The combined unit is fabricated by incorporating a GDLbearing serpentine flow field channels into a recessed flat conductiveplate surface. The channel landings formed on the GDL contacts thesurface of the flat plate. There is no bonding between the GDL and theflat plate. The recessed surface of the flat plate helps to hold the GDLin place and prevents the GDL from getting dislocated during stacking ofthe cells. However, due to the absence of electrically conductivebonding between the landings of the GDL and the surface of the plate,the resultant conductivity between these two components remainsdependent on the stack compression force. Higher force will result inbetter contact and hence lower resistive loss between the GDL and thesurface of the plate. Similar concepts have also been disclosed in U.S.Pat. Nos. 5,252,410 and 5,300,370.

A method for improving the conductivity between the GDL and separatorplates and to prevent the resilient GDL from sinking into the open-facedflow channel under stack compression force has been disclosed in U.S.Pat. No. 6,007,933′. An electrically conductive support member withfirst and second sides is placed between the resilient GDL and theseparator plate face. The support member is formed with a plurality ofopenings extending between the first and second sides. The first side ofthe support member abuts the separator plate face. The second side ofthe support member abuts the resilient GDL and prevents the GDL fromentering the open-faced flow channel under a compressive force appliedto the stack assembly. Thus, the support member acts as an additionallayer between the plate and GDL, however, it contributes to the overallresistive loss of the stack assembly.

Another method that tries to develop a conductive contact between theGDL and the current collector plate is to use different thermoplasticadhesive films between the GDL and the plate. This is disclosed in E.P.patent number 0,330,124. Thin thermoplastic films are placed on the GDLthat possess flow field landing surfaces. The GDL is heat pressedagainst the flat current collector plate to create an integratedlaminated structure. The thermoplastic film gets impregnated into thepores of the GDL and the pores on the surface of the current collectorplate to create a permanent bond between the two components. Afterbonding, the conductivity between the GDL and the plate becomesindependent of the compression force applied to the stack.Unfortunately, being an electrical insulator, the thermoplastic adhesivefilm does not provide optimum conductivity between the GDL and theplate.

During operation of the electrochemical PEM cell stack, current outputor utilization is limited by several factors. Ohmic resistance is themost significant limiting factor. Ohmic resistance is created withineach PEM cell and by the interface between each PEM cell. This isdescribed in a DOE Report “Understanding of Carbonate Fuel CellResistance Issues for Performance Improvement” Contract#DE-AC21-90MC27168. Further limitations are imposed by the backpressurecreated as the gases flow through each PEM cell when the GDL sinks intothe flow field channels of the separator plate. Large current outputrequires high flow rates, which result in increased backpressure if theGDL occupies the channels unnecessarily. High backpressure tends tocontribute to reactant gas leakage and hence to a mass transportproblem, which reduces the overall stack efficiency.

There, therefore, remains a need to provide a process for improving thenature and extent of contact between the GDL and flow field separatorplates while not increasing the stack compression force of the fuel cellstack. This unitized GDL and plate component will be advantageous forproviding continuous support of the fuel cell MEA, homogenizeddiffusion, permeability and GDL integrity, preventing the GDL fromsinking into the flow field channels, providing better electricalcontact between the GDL and the plate, and reducing the amount of stackcompression force for satisfactory electrical conductivity between theMEA and the combined separator plate and GDL.

SUMMARY OF THE INVENTION

The present invention provides a process for joining the GDL to flowfield separator plates to form an electrochemical cell component.

According to one aspect of the invention there is provided a process forjoining a gas diffusion layer to a separator plate of an electrochemicalcell, wherein the gas diffusion layer comprises a porous body, and theseparator plate comprises at least one landing surface formed on asurface of the separator plate, and the separator plate and landingsurface comprising a polymer and conductive filler, the processcomprising the step of welding the landing surface to the gas diffusionlayer by impregnating some of the polymer on the landing surface withina portion of the porous body.

In one embodiment of the invention resistance welding is used to jointhe GDL to the flow field separator plate. In another embodiment,vibrational welding is used.

The preferred embodiments of the present invention can provide manyadvantages. For example, the process of the present invention improvesthe electrical contact between the GDL and the landing surfaces of theflow field separator plates and provides uniform conductivity across theGDL and plates. It results in negligible resistive loss between the GDLand the plates, leading to better overall performance of theelectrochemical cell stack. Other advantages include the cell componentproviding continuous support for the MEA, allowing for homogenizeddiffusion and permeability of the reactants and product fluids, allowingfor uniform electrical contact between the GDL and the plate, preventingthe sinking of GDL material into the open channels of the flow fieldplate thus keeping the reactant gas flow through the channelsunaffected, and reducing the amount of stack compression needed forsatisfactory electrical conductivity between the GDL and flow fieldplate.

Numerous other objectives, advantages and features of the process willalso become apparent to the person skilled in the art upon reading thedetailed description of the preferred embodiments, the examples and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the present invention will be describedwith reference to the accompanying drawings in which like numerals referto the same parts in the several views and in which:

FIG. 1 a is an exploded perspective view of a typical polymerelectrolyte membrane fuel cell;

FIG. 1 b is an exploded perspective view of a typical polymerelectrolyte membrane fuel cell as part of a fuel cell stack;

FIG. 2 a is a partial side perspective view of a flow field plate andgas diffusion layer;

FIG. 2 b is a close up view of flow field channels and landing surfaceson a flow field plate;

FIG. 3 is a schematic drawing of a weld created between a flow fieldplate and a gas diffusion layer in accordance with a preferredembodiment of the present invention; and

FIG. 4 illustrates the relationship between resistivity and compressionpressure for a gas diffusion layer joined to a graphite/polymercomposite flow field plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will now be describedwith reference to the accompanying figures.

As shown in FIG. 1 a, a typical polymer electrolyte membrane fuel cellcomprises a MEA disposed between two flow field separator plates 5. TheMEA includes a catalyst-coated membrane 10 between two gas diffusionlayers (GDLs) 15. The GDLs 15 are adjacent to flow field plates 5, whichform the outer layers of the fuel cell.

The flow field plates 5 comprise at least one flow field channel 20 thatallows gas or liquid to flow within the electrochemical fuel cell. Theflow field plates 5 typically carry either fuel or oxidant depending onthe design of the electrochemical cell or electrochemical cell stack.

In a preferred embodiment, the present invention provides a process forjoining the GDL 15 to the flow field separator plates 5 by partiallyimpregnating the polymeric composite plate material of the flow fieldplate 5 into the pores of the GDL 15.

Impregnation of the composite material of the flow field plate 5, inparticular the polymer component of the plate, is facilitated by theconfiguration and composition of the flow field plates 5. As shown inFIGS. 2 a and 2 b, the flow field channels 20 are configured withlanding surfaces 25, which are raised surfaces that form the top barrierwalls of the flow field channels 20. The dimensions of the landingsurfaces 25 may vary according to the particular electrochemical celldesign. In a preferred embodiment, both the width and height of thelanding surfaces 25 are from 0.1 mm to 8.0 mm, preferably from 0.8 mm to1.5 mm.

The separator plates 5 are generally moulded from a compositioncomprising a polymer resin binder and conductive filler, with theconductive filler being preferably graphite fibre and graphite powder.The polymer can be any thermoplastic polymer or any other polymer havingcharacteristics similar to a thermoplastic polymer. The thermoplasticpolymers can include melt processible polymers, such as Teflon® FEP andTeflon® PFA, partially fluorinated polymers such as PVDF, Kynar®, KynarFlex®, Tefzel®, thermoplastic elastomers such as Kalrez®, Viton®,Hytrel®, liquid crystalline polymer such as Zenite®, polyolefins such asSclair®, polyamides such as Zytel®, aromatic condensation polymers suchas polyaryl(ether ketone), polyaryl(ether ether ketone), and mixturesthereof. Most preferably, the polymer is a liquid crystalline polymerresin such as that available from E.I. du Pont de Nemours and Companyunder the trademark ZENITE®. A blend of 1 wt % to 30 wt %, morepreferably 5 wt % to 25 wt % of maleic anhydride modified polymer withany of the above-mentioned thermoplastic polymers, partially fluorinatedpolymers and liquid crystalline polymer resin and their mixture can alsobe used as binding polymer.

The graphite fiber is preferably a pitch-based graphite fiber having afiber length distribution range from 15 to 500 μm, a fiber diameter of 8to 15 μm, bulk density of 0.3 to 0.5 g/cm³ and a real density of 2.0 to2.2 g/cm³. The graphite powder is preferably a synthetic graphite powderwith a particle size distribution range of 20 to 1500 μm, a surface areaof 2 to 3 m²/g, bulk density of 0.5 to 0.7 g/cm³ and real density of 2.0to 2.2 g/cm³. Further details regarding the composition of the separatorplates 5 are described in U.S. Pat. No. 6,379,795 B1, which is hereinincorporated by reference.

In a preferred embodiment of the present invention, the separator plates5 are molded from a composition as described in co-pending of PCT patentapplication no. PCT/CA03/00202 filed Feb. 13, 2003, the completespecification of which is hereby incorporated by reference. Thecomposition includes from about 1 to about 50% by weight of the polymer,from about 0 to about 70% by weight of a graphite fibre filler havingfibres with a length of from about 15 to about 500 microns, and from 0to about 99% by weight of a graphite powder filler having a particlesize of from about 20 to about 1500 microns. Preferably, the compositioncomprises:

-   -   a. from about 1 wt % to about 50 wt % of ZENITE® 800 aromatic        polyester resin;    -   b. from about 0 wt % to about 70 wt % of pitch-based graphite        fiber (fiber length distribution range: 15 to 500 micrometre;        fiber diameter: 8 to 10 micrometre; bulk density: 0.3 to 0.5        g/cm³; and real density: 2.0-2.2 g/cm³); and    -   c. from about 0 wt %/o to about 99 wt % graphite powder        (particle size distribution range: 20 to 1500 micrometre;        surface area: 2-3 m²/g; real density: 2.2 g/cm³).

The GDLs useful in the present invention are generally made of porous,electrically conductive non-corrosive material, such as woven andnon-woven carbon cloth or carbon paper, which are available under thetrade names SGL, Lydall Technimat, Toray, Ballard Aucarb, MitsubishiRayon, Kureha isotropic, Zoltek, Freudenberg nonwovens, E-TEK “E-LAT”,Spectracorp Spectracet, and Conoco mesophase. The GDLs may havemicroporous layers deposited on their surfaces for controlling the shapeand size of the pores, which is important to control the gaspermeability of the GDLs. These GDLs can be treated with Teflon® forinducing hydrophobicity, which is useful for electrochemical celloperation. The GDLs provide uniform fuel and oxidant distribution to thecatalyst-coated membrane and facilitates the transport of product waterfrom the catalyst layers to the flow field separator plates. The GDLsalso provide electrical contact between the catalyst layers and plates,which helps in the harvesting of electrons from the electrochemicalreaction in the electrocatalyst layers.

To join the GDL 15 to the flow field plate 5, the GDL 15 is welded tothe landing surfaces 25 using suitable joining techniques such asresistance welding and vibration welding. However, other techniques suchas ultrasonic welding, laser welding, heat lamination, or hot bondingtechniques may also be used.

The general process for resistance welding of two electricallyconductive polymer composites is disclosed in U.S. Pat. No. 4,673,450,which is hereby incorporated by reference. However, its application toelectrochemical cells and to a process of welding an electricallyconductive polymer composite material with porous non-polymeric materialhas not yet been explored.

In the resistance welding process, an alternating or direct electricalcurrent is used to create welds between the landing surfaces 25 and theGDL 15, thereby joining the landing surfaces 25 to the GDL 15. Theelectrical current is passed between the GDL 15 and flow field separatorplate 5 while bringing the landing surfaces 25 and GDL 15 together.Pressure may also be applied to the GDL 15 and separator plate 5 to keepthe GDL 15 and flow field plate 5 together. Preferably, during theresistive welding process, a constant pressure is applied to the GDL 15to hold the GDL 15 against the landing surfaces 25.

As the electrical current flows through the flow field separator plate 5and GDL 15, the contact areas between the GDL 15 and landing surfaces 25experience a relatively higher resistance, resulting in the productionof localized heat at the contact areas. This localized heat causes thepolymer component of the landing surfaces 25 to melt, allowing theconductive filler components of the landing surface 25 to establishdirect contact with the carbon matrix of the GDL 15, thereby creating acontact region. When the polymer component of the landing surfaces 25 ismolten, the flow of electrical current is stopped, thus also stoppingthe production of localized heat. As pressure continues to be appliedbetween the GDL 15 and landing surfaces 25, the molten polymer componentof the landing surfaces 25 impregnates into the pores of the GDL 15.With continued pressure applied, the molten polymer component of thelanding surfaces 25 then solidifies into the pores of the GDL 15 andaround the contact region as the polymer cools, thereby fusing the GDL15 to the flow field separator plates 5 at the landing surfaces 25.Since localized heat production stops when the electrical current iswithdrawn, the temperature of the separator plates 5 drops quickly to atemperature well below the glass transition temperature of the polymer.As a result, the molten polymer hardens and the fused areas between thelanding surfaces 25 and GDL 15 form a permanent weld 40 (see FIG. 3).

The electrical current can be applied directly to the flow fieldseparator plates 5 and GDL 15 using electrodes. The amperage, voltage,design pressure and span of electrical current flow will vary dependingon the polymer resin material used, the surface area of the landingsurfaces 25, the size of the pores of the GDL 15 and the degree ofmelting at the landing surfaces 25. However, in a preferred embodiment,the applied electrical current is between about 0.01 amperes/mm² andabout 5 amperes/mm², preferably between about 0.8 and about 1.1amperes/mm², and its voltage is between about 1 and about 10 volts,preferably between about 3 and about 5 volts. The electrical current isapplied for a time of from about 0.1 to about 10 seconds, preferablyfrom about 0.5 to about 4 seconds and the applied pressure is betweenabout 1 and about 200 psig, preferably between about 10 and about 120psig, more preferably between about 30 and about 70 psig.

The electroconductive integrated cell component formed by joining theGDL 15 to the flow field separator plates 5 can be used to create a fuelcell that includes the GDL 15 permanently fused to the plates 5 withinthe fuel cell. This fuel cell design will have significant advantagessuch as improved efficiency, low stack clamping force and decreasedproduction time.

With the vibration welding technique, a vibration welding machine isused to create a vibrational force amongst and between the GDL 15 andthe flow field separator plates 5. The GDL 15 and plates 5 are broughttogether and placed so that the GDL 15 is in contact with the landingsurfaces 25. The vibrational force can be applied to both the GDL 15 andthe flow field separator plate 5, or either one of the GDL 15 or plate 5while keeping the other stationary.

The continued vibrational force on the GDL 15 and the flow fieldseparator plates 5 causes the contact area between the GDL 15 andlanding surfaces 25 to become frictionally engaged, resulting in theproduction of localized heat which melts the polymer component presentin the composite material at the landing surfaces 25. When thevibrational force is reduced or is stopped, localized heat production isdiminished or eliminated and the GDL 15 and the landing surfaces 25 arecooled, solidifying the localized molten polymer and fusing the areabetween the GDL 15 and the landing surfaces 25. Pressure is preferablyapplied to the GDL 15 and the flow field plate 5 during cooling to causethe molten polymer composition to impregnate into the pores of the GDL15. The preferred pressure applied is between about 1 and about 200psig, preferably between about 10 and about 120 psig, more preferablybetween about 30 and about 70 psig.

The landing surfaces 25 of the flow field separator plates 5 areconfigured so that during vibration welding of the plates 5 and GDL 15only the landing surfaces 25 are in contact with the GDL 15, while therest of the plate 5 is not in contact with the GDL 15. As shown in FIG.3, this configuration allows the polymer component in the landingsurfaces 25 to melt during vibration.

The amplitude, frequency and application time of the vibrational forceapplied to the GDL 15 and the flow field separator plate 5 determinesthe extent to which the landing surfaces 25 will fuse with the GDL 15.In a preferred embodiment, the vibrational is applied for a time ofabout 3 to about 100 seconds, at a frequency of about 100 to about 500cycles per second and amplitude of about 0.5 mm to about 5 mm. It willbe apparent to a person skilled in the art that the amplitude, frequencyand vibrational timing of the vibrational welding process is designed tocomplement the impregnation of the polymer of the landing surfaces 25into the pores of the GDL 15.

The quality of the weld between the GDL 15 and landing surface 25created by the welding process can further be improved by providing apolymer rich material or pure polymer layer to the landing surfaces 25of the flow field separator plate 5. The plate 5 may therefore bepolymer rich at a localized area on the top surface of the landingsurfaces 25. In a preferred embodiment, the localized area is 0.002″ to0.100″ thick and more preferably 0.020″ thick. This localized areacomprises between about 25 wt % and about 100 wt % polymer, preferablybetween about 50 wt % and about 100 wt % polymer, and most preferablyabout 100 wt % polymer.

The following examples illustrate the various advantages of thepreferred method of the present invention.

EXAMPLES

Composite separator plates comprising 25% Zenite® 800, 55% Thermocarb®graphite powder and 20% graphite fiber were welded to three differentgas diffusion layers, namely E-TEK “E-LAT” carbon cloth, Zoltek®, andBallard Aucarb®. E-TEK® carbon cloth comprises a weaved carbon clothmaterial and has a regular metric structure. Zoltek® cloth comprises awoven carbon cloth material. It has an irregular surface porosity.Ballard Aucarb® comprises a small porous structure and has increasedrigidity when compared with E-Tek® and DeNora Zoltek®.

A jig was used to apply a direct current mediated by two electrodesdirectly to the composite plate and gas diffusion layer. A weldingmachine was used as a power source. The jig also applied and controlledthe pressure on the composite plate and gas diffusion layer. A gascylinder was used as the pressure source.

Example 1

A composite plate bearing a landing surface was joined to a gasdiffusion layer comprising porous E-Tek® carbon cloth. The landingsurface of the plate had a length of 60 mm, a width of 20 mm and athickness of 4 mm. The composite plate and the gas diffusion layer wereplaced in the jig (for Butt welding position) and a 70-ampere (70 A)current was passed through the composite plate and gas diffusion layerfor 3 seconds. A pressure of 60 psig was applied to hold the gasdiffusion layer against the landing surfaces of the plate. Afterapplication of the electrical current, the gas diffusion layer was heldagainst the landing surface of the plate using the weld pressure of 60psig. Once the integrated gas diffusion layer and plate were cooled toroom temperature, the pressure was released and electrical conductivityof the integrated component was measured.

Example 2

A composite plate similar to the one described in Example-1 was used tojoin to a gas diffusion layer comprising porous Zoltek® carbon clothobtained from DeNora. Zoltek® gas diffusion layer was placed on thelanding surface of the plate and both were then placed in the resistivewelding jig and held at a pressure of 55 psig. A 90-ampere electricalcurrent was applied through the electrodes for 3.5 seconds. The pressurewas held at 55 psig until the joined cell component was cooled down toroom temperature. Once cold the pressure was removed and theconductivity of the joined electroconductive component was determined.

Example 3

A composite plate similar to the one described in Example-1 was used tojoin to a gas diffusion layer comprising porous Zoltek® carbon clothobtained from DeNora. Zoltek® gas diffusion layer was placed on thelanding surface of the plate and both were then placed in the resistivewelding jig and held at a pressure of 55 psig. A 60-ampere electricalcurrent was applied through the electrodes for 3.0 seconds. The pressurewas held at 55 psig until the joined cell component was cooled down toroom temperature. Once cooled, the pressure was removed and theconductivity of the joined electroconductive component was determined.

FIG. 4 compares the resistivity of the joined Zoltek®-composite platesystem to a system where the Zoltek® carbon cloth is not welded to theplate. From FIG. 4, it will be noted that the resistivity of the weldedcomponent remains essentially constant as compression pressureincreases, and in fact remains constant at 1.24Ω when the compressionforce is increased from 0.5 MPa to 2.0 MPa. In contrast, the resistivityof the system where the gas diffusion layer is not welded to the platedecreases significantly as the compression pressure increases to about0.8 MPa. Then, the resistivity remains fairly constant at about 1.4 MPaas the pressure is increased from 0.8 to 2.0 MPa.

Thus, from FIG. 4, it will be seen that the resistivity of the componentwelded in accordance with a preferred embodiment of the process of thepresent invention is less than the resistivity of the non-welded system(1.24Ω compared to 1.4Ω). As well, the resistivity of the weldedcomponent is essentially not dependent on the compression pressurewhereas the resistivity of the non-welded system is dependent oncompression pressure, especially at pressures less than 0.8 MPa.

Example 4

Ballard Aucarb® gas diffusion layer was joined to the composite platedescribed in Example-1 using resistive welding method as described inExamples 1 and 2. A welding current of 90-ampere was applied for 3.5seconds to the gas diffusion layer and plate while a pressure of 50 psigwas applied. Once the plates were cooled to room temperature, thepressure was removed and the integrated Ballad Aucarb® gas diffusionlayer and composite plate was subjected to conductivity measurements.

As discussed earlier, gas diffusion layers comprising different surfacemorphology and pore structure require different welding conditions. Ineach case, welding of the four different gas diffusion layers to thelanding surface was achieved. Table 1 compares the variation in weldingconditions for four different gas diffusion layers. TABLE 1 WeldingParameters and Conductivity Gain E-TEK Ballard Welding Carbon ClothZoltek ® Zoltek ® Aucarb ® Parameters (Example 1) (Example 2) (Example3) (Example 4) Current 70 90 60 90 (amp) Pressure 60 55 55 50 (psig)Weld Time 3.0 3.5 3.0 3.5 (s)

Although the present invention has been shown and described with respectto its preferred embodiments and in the examples, it will be understoodby those skilled in the art that other changes, modifications, additionsand omissions may be made without departing from the substance and thescope of the present invention as defined by the attached claims.

1. A process for joining a gas diffusion layer to a separator plate ofan electrochemical cell, wherein the gas diffusion layer comprises aporous body, and the separator plate comprises at least one landingsurface formed on a surface of the separator plate, and the separatorplate and landing surface comprising a polymer and conductive filler,the process comprising the step of welding the landing surface to thegas diffusion layer by impregnating some of the polymer on the landingsurface within a portion of the porous body.
 2. The process of claim 1,wherein the welding step is selected from the group consisting ofresistance welding, vibrational welding, ultrasonic welding, laserwelding, heat lamination, and hot bonding techniques.
 3. The process ofclaim 2, wherein the welding step is resistance welding.
 4. The processof claim 3 wherein resistance welding comprises the further steps of:(a) placing the landing surface in contact with the gas diffusion layer;(b) applying an electrical current between the gas diffusion layer andthe separator plate to produce localized heat at the landing surfacesufficient to melt the polymer in the landing surface and produce moltenpolymer; (c) applying pressure to the landing surface and gas diffusionlayer to allow the molten polymer to impregnate into the portion of theporous body; and (d) ceasing to apply the electrical current to allowthe molten polymer to cool and solidify. 5-22. (canceled)
 23. Theprocess of claim 4, wherein the electrical current is between about 0.01amperes/mm² and about 5 amperes/mm², its voltage is between about 1 andabout 25 volts and the current is applied for a time from about 0.5 toabout 100 seconds.
 24. The process of claim 23, wherein the electricalcurrent is between about 0.8 and about 1.1 amperes/mm².
 25. The processof claim 4 wherein the pressure applied is between about 1 and about 200psig.
 26. The process of claim 25 wherein the pressure applied isbetween about 10 and about 120 psig.
 27. The process of claim 25 whereinthe pressure applied is between about 30 and about 70 psig.
 28. Theprocess of claim 4 wherein the electrical current is applied usingexternal electrodes.
 29. The process of claim 2, wherein the weldingstep is vibration welding.
 30. The process of claim 29, wherein thevibration welding step comprises the further steps of: (a) placing thelanding surface in contact with the gas diffusion layer; (b) applying avibrational force between the separator plate and the gas diffusionlayer to produce localized heat at the landing surface sufficient tomelt the polymer at the landing surface; (c) applying pressure to thelanding surface and gas diffusion layer to allow the molten polymer toimpregnate into the portion of the porous body; and (d) ceasing to applythe vibrational force to allow the molten polymer to cool and solidify.31. The process of claim 30 wherein the vibrational force is applied ata frequency of between about 100 and about 500 cycles per second for atime from about 3 to about 100 seconds at an amplitude of between about0.5 and about 5 mm.
 32. The process of claim 30 wherein the pressureapplied is between about 1 and about 200 psig.
 33. The process of claim32 wherein the pressure applied is between about 10 and about 120 psig.34. The process of claim 1 wherein the polymer is a thermoplasticpolymer selected from the group consisting of melt processible polymers,partially fluorinated polymers, thermoplastic elastomers, liquidcrystalline polymers, polyolefins, polyamides, aromatic condensationpolymers, and mixtures thereof.
 35. The process of claim 34, wherein thepolymer is a blend of about 1 wt % to about 30 wt %, preferably about 5wt % to about 25 wt %, of maleic anhydride modified polymer with thethermoplastic polymer, partially fluorinated polymers and liquidcrystalline polymer or mixtures thereof.
 36. The process of claim 1wherein the conductive filler is graphite fiber or graphite powder. 37.The process of claim 1 wherein the landing surface comprises a polymerrich outer layer.
 38. The process of claim 37, wherein the polymer richouter layer comprises between about 25 wt % and about 100 wt % polymer,preferably between about 50 wt % and about 100 wt % polymer, and mostpreferably about 100 wt % polymer.
 39. An electrochemical cell componentcomprising a gas diffusion layer welded to a separator plate using theprocess of claim
 1. 40. An electrochemical cell comprising a gasdiffusion layer welded to a separator plate using the process ofclaim
 1. 41. An electrochemical cell comprising the electrochemical cellcomponent of claim
 39. 42. An electrochemical cell stack comprising aplurality of the electrochemical cells of claim
 41. 43. Anelectrochemical cell component of claim 39, wherein the electrochemicalcell component has a resistivity less than a resistivity of a systemcomprising a gas diffusion layer that is not welded to a plate.
 44. Anelectrochemical cell component of claim 39, wherein the surface of theseparator plate comprises open flow field channels and the gas diffusionlayer does not sink into the open flow field channels.